Writing a century and a half before the birth of Christ, the Greek historian Polybius observed “nowadays all over Greece such a diminution in natality and in general manner such depopulation that the towns are deserted and the fields lie fallow. Although this country has not been ravaged by wars or epidemics, the cause of the harm is evident: by avarice or cowardice the people, if they marry, will not bring up the children they ought to have. At most they bring up one or two. It is in this way that the scourge before it is noticed is rapidly developed.”

He concluded by urging his fellow Greeks to return to their historic love of family and children. “The remedy is in ourselves,” he wrote. “We have but to change our morals.” His advice, unfortunately, went largely unheeded.

The demographic winter of the Greek city-states led to economic stagnation and military weakness, which in turn invited invasion and conquest. After a century of increasing dominance in the Eastern Mediterranean, Rome finally annexed the Greek city-states in 146 B.C.

Will a Europe in the grip of a similar demographic winter come to a similar unhappy end? Certainly Europeans of today, like the Greeks of old, are barely having children. The birthrate across the entire continent is far below the replacement level of 2.1 children per couple. Italy, Spain, Austria, and Germany have total fertility rates, or TFRs, of only 1.4 or so, while Poland and Russia languish at 1.32 and 1.2 respectively. The more or less generous child allowances these countries pay the prolific has scarcely caused these numbers to budge. The birth dearth continues to widen.

Meanwhile, adherents of pro-family sects such as Islam are moving in, having children, and repopulating historic Christendom. Is this process likely to continue? And to what end?

Most Muslim countries in North Africa and the Middle East have fertility rates two or three times as high as Europe. Afghanistan and Somalia, whose fertility rates are above 6 children (6.62 and 6.4 respectively), may be outliers. But other Middle Eastern countries with above-replacement TFRs include Iraq at 4.86, Pakistan at 3.65, and Saudi Arabia at 3.03. Even immigrants from the most Westernized Muslim countries such as Turkey and Tunisia average nearly twice as many children as the extant populations of most European countries.

While falling fertility may be humanity’s general fate, it is this differential fertility that will determine Europe’s destiny. Although the birthrates of Muslim immigrants to Europe are far lower than they were just a generation ago, they are still far more open to life than highly secularized Europeans. Moreover, these immigrants, once in place in Germany, Italy, Spain, etc., tend to maintain their relatively high fertility for a generation.

As a result of this potent mix of immigration and procreation, the number of Muslims will continue to grow. Europe as a whole, some demographers suggest, will have a majority Muslim population by 2100.

What a strange twist of history! Over the centuries, various Muslim armies have repeatedly attempted to conquer Europe. Time and time again, at Tours, Vienna, at Lepanto, at Malta, they were thrown back. Yet now what their forebears were unable to accomplish by force, their distant descendants will achieve by peacefully winning the Battle of the Cradle.

Whether they will be radicalized or secularized Muslims is the central question. If they are radicalized, then we can expect efforts to impose Sharia law in country after country, along with the growing persecution of the Christian minority. Catholics in Germany, for example, may come to be treated in largely the same way that Coptic Christians in Egypt have been for the last few centuries, that is to say, as second-class citizens, to be maligned, taxed and beaten almost at will.

If, on the other hand, the second- and third-generation Muslims are largely secularized, then the Christian minority will be, presumably, treated somewhat better, though still subject to some level of discrimination. As everyone knows by now, the Secular Left preaches a tolerance that it generally does not practice.

Either way, believers in once-Christian Europe will probably find themselves living in what might be called a pre-Constantine moment. In others words, they will be living under regimes that punish, even persecute, them for their beliefs.

At the present moment, Europeans still control their own destiny. As Polybius, were he alive today, would surely remind them: “The remedy is in yourselves. You have but to change your morals.”

Steven W. Mosher is the President of the Population Research Institute.